Stickies are already baked into Mac OS X, but Stick 'Em Up is like Stickies on steroids. Stick 'Em Up offers a wide range of features like customizable font styles, sizes, colors, and a slew of categorization features:

Stick 'Em Up lets you create notes in different colors and sizes that can contain rich text and graphics. With Stick 'Em Up you group your notes in to categories, only the notes in a selected category are shown on screen. You can navigate your categories via the application menu, keyboard commands, or with a small categories list window.